Recently Photoacoustic Spectroscopy (PAS) has emerged as a significant new tool for the study of light absorption in solids and liquids. It is especially useful in cases where strong light scattering or opacity in the absorbing sample makes the application of conventional optical absorption methods difficult. It also has application in studying energy conversion processes which compete with thermalization in the sample for the energy in the absorbed light. At its current state of development PAS-derived information has been primarily qualitative in nature with the principal experimental result being the determination of the relative spectral amplitude of the PAS signal as a function of wavelength. Detailed considerations of the responsivity of fluid filled photoacoustic cells have shown that the measured PAS signal depends on thermal diffusion in four regions which constitute the complete cell, viz. the light-absorbing sample, the support or backing which holds the sample, the gas or liquid pressure transducing medium, and the cell walls and light-admitting window. In keeping with the diffusion character of the basic processes responsible for generation of the effect, the PAS signal is scale dependent, that is, it depends on the size of each region of the photoacoustic cell relative to the thermal diffusion lengths, .mu..sub.i =[2a.sub.i /.omega.].sup.1/2, in each region, where a is the thermal diffusivity, and .omega. the angular modulation frequency. The thermal diffusion lengths in both the cell and sample depend on the modulation frequency and hence the cell responsivity is frequency dependent. Since the basic objective of the measurement is the determination of the sample properties, including the optical absorption co-efficient, these cell-dependent effects are experimental impediments which must be removed if the sample properties are to be determined. Fortunately, for fixed modulation frequency operation in a single cell, these complications do not interfere with the measurement of the relative sample absorption spectrum. This type of measurement has characterized the bulk of prior art PAS measurement methods.
U.S. Pat. Nos. 3,911,276; 3,893,771; 3,820,901; and 3,700,890 all describe photoacoustic spectrometers which measure relative absorption spectra without reference to absolute values of energy. The problems of cell structure dependency and the inability to translate data between cells remain unsolved by the prior art techniques. Another U.S. Pat. No. 3,948,345 employs a metal black sample in a standard cell wherein pulsed light is split between the standard cell and a test cell including a test sample. A comparison between the pressure waves generated in the standard cell and the pressure waves generated in the test cell is made to determine the relative absorption in the test cell. The reference does not provide an absolute measurement independent of cell characteristics. The conductivity of the black sample is not important in the reference; the black sample is not used for self-calibration.